There is a wide use of amplifiers in today's electronic circuits. One type of amplifier, known as a “pre-amplifier,” is typically used to amplify (“preamplify”) an unrefined, weak signal that is received from some source, such as a microphone transducer. In a microphone usage, the comparatively small input voltage signal is provided by a microphone transducer to an amplifier referred to as a microphone pre-amplifier. The microphone pre-amplifier amplifies the input analogue audio signal to provide a larger-amplitude “preamplified” output signal that has the same waveform as the input signal, within a particular tolerance.
The preamplified signal is then passed on to a further amplification and/or signal processing stages. For example, the pre-amplified signal may be processed by further components, such as an automatic gain control (AGC) circuit or other components or stages. The pre-amplified signal is often further amplified by an amplifier, to drive an output device, such as a loudspeaker. Other uses may also be made of the preamplified signal.
As the voltage signal input into a pre-amplifier is weak, it is important for microphone pre-amplifiers, as well as other amplifiers that amplify weak input signals, to have a very good noise performance. If not, the noise, which may be significant in comparison to the small input signal, may dominate the amplified output signal. Noise arises in a variety of ways. For example, in amplifiers (including pre-amplifiers) implemented as part of an integrated circuit (IC) having a substrate, noise can arise from comparatively noisy sections of the substrate and thus adversely affect the amplifier portions of the IC.
Noise may also be generated when unavoidable parasitics, associated with silicon ICs, provide numerous paths for unwanted disturbances to couple into the signal path of an analog circuit via the substrate, the power supply rails, the ground lines, and/or even directly from nonideal components. Noise may accompany the input signal if the input lines that deliver the input signal from the microphone transducer pick up noise from a source external to the IC. Such noise may emanate from, for example, a noisy power supply that powers a transducer such as a microphone. Feedback resistors may also introduce thermal noise into the input signal path. The input stage of the amplifier itself may also introduce thermal noise and so-called “flick” noise.
It is also known that sources of noise that are not intrinsic to the silicon components are often reduced by employing well-known techniques, such as: shielding (external to the IC), proper decoupling of power supplies, proper layout of sensitive signals and grounds, isolating active components from noisy substrate. However, the noise sources associated with each silicon component (resistor, transistor), which we can refer to as intrinsic noise sources (thermal noise and flick noise), cannot be reduced unless we accept a very large penalty in terms of current consumption and die size, which is clearly unacceptable for low cost, portable applications.
Noise may thus be introduced into the signal path of an amplifier (or preamplifier), and be amplified along with the input signal, thus causing the amplified (pre-amplified) output signal to be a distorted representation of the input waveform. Such disturbances and distortions can accumulate, potentially leading to serious loss in signal-to-noise ratio and dynamic range of the microphone design.
Referring now to FIG. 1, a classical microphone amplification arrangement 100 is illustrated. The classical microphone arrangement 100 comprises a microphone transducer 125. Notably, the remaining circuitry to support the use of the microphone transducer is inherently dictated by the specifications set by the microphone transducer manufacturer. In particular, the microphone transducer manufacturer specifies a voltage output signal to be supplied to bias the microphone at location 120, as well as the sensitivity (gain) of the microphone. Consequently, this dictates the bias resistor Rb 115 value and specification, as well as the microphone bias IC 110.
The microphone voltage Vmic at location 120 is created by the microphone current and is set by the dc bias, the value of Rb 115 and the dc current drawn by the microphone transducer 125.
The microphone bias IC 110 is a small low-noise voltage regulator, which is configured to provide, say, a regulated 2.1V in accordance with the microphone transducer manufacturer's specification of supply voltage. The microphone bias IC 110 is supplied by a further low noise reference voltage (VAG) 105. The low noise reference voltage (VAG) 105 is equivalent to a voltage regulator with limited current capability. Often, this voltage level is set to half of the supply voltage, to facilitate maximum swing of the voltage being modulated.
The standard microphone amplifier circuit is a voltage-to-voltage (V2V) circuit, as the microphone current Imic is first transformed into a voltage Vmic and the amplification Vout/Vmic involves signals in a voltage form. The microphone transducer and associated bias circuitry is typically coupled to a microphone amplifier integrated circuit (IC), via a large external capacitance 130 and an IC pin 135.
The microphone amplifier IC comprises an input resistor Rin 140, a preamplifier 145 and feedback network comprising a feedback resistor 155. The amplified audio output signal 160 is output from the pre-amplifier 145.
The standard arrangement in a V2V microphone amplifier is that the preamplifier is configured as an inverting amplifier, which amplifies the voltage at the microphone with a gain Rf/Rin.
This classical amplification arrangement is referred to as a voltage to voltage (V2V) amplification because the microphone signal Imic resulting from the acoustic-to-electric conversion is first transformed into a voltage, before being amplified by the preamplifier 145. The global sensitivity (gain) of the microphone and pre-amplifier is illustrated below in equation [1].
                                          V            out                                I            mic                          =                                                            V                out                                            V                mic                                      ·                                          V                mic                                            I                mic                                              =                                                    R                                  fv                  ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                  v                                                            R                                  i                  ⁢                                                                          ⁢                  n                                                      ·                          (                                                                    R                                          i                      ⁢                                                                                          ⁢                      n                                                        //                                      R                    b                                                  //                                  R                                      a                    ⁢                                                                                  ⁢                    c                                                              )                                                          [        1        ]            
In, for example, the field of photonics, it is known to use a current-to-voltage (I2V) circuit to convert current produced by photo-diodes into a voltage. Here, a pre-amplifier and feedback path is also used. However, no input resistance is used.
Referring now to FIG. 2, a classical microphone amplification system 200 is illustrated. The classical microphone amplification system 200 is suitable for mobile phone use and comprises a series of audio/microphone inputs 210, such as microphone input, auxiliary microphone and an external microphone. Each of the microphone inputs 210 are input to a respective low-noise pre-amplifier arrangement 220. Each of the low-noise pre-amplifiers is illustrated as a V2V conversion arrangement, which has evolved to be the de-facto standard in microphone amplification.
The low-noise pre-amplifier arrangement 220 has respective output ports to an analogue multiplexer 230 that is under control of the mobile phone controller to select the microphone input to be processed and transmit.
The analogue multiplexer 230 has a single output to a programmable gain amplifier 240 to amplify the audio signal to a level that is sufficient for processing and yet is undistorted. The programmable gain amplifier 240 outputs the amplified audio signal to a single-to-differential amplifier (converter) plus anti-aliasing filter for subsequent data conversion by an analogue-to-digital converter 250.
However, although the aforementioned microphone amplification system uses low-noise pre-amplifiers, the product manufacturers who utilize such microphone amplification systems, such as mobile phone manufacturers are requiring improvements in reducing noise. This is particularly the case for provision of future technologies and features where audio sensitivity to noise has been identified as a key user requirement.
Thus, there exists a need to improve the noise performance in microphone amplification systems. In addition, there is a need to support existing microphone amplifier applications that use the standard V2V approach. Furthermore, any solution needs to be capable of providing adequate electro-magnetic interference (EMI) protection.